My suggestion... Connect the black wire to your marker circuit and forget about the red wire. Problem solved. You have a clearance marker light on the back and you don't have to worry about isolating the turn signals from each other.
DING, DING, DING, DING!!! We have a WINNA!!!! That's what I got to thinking yesterday afternoon. To heck with the Red wire, this just overly complicates things and just having it as a tail light will work fine. I don't have the light bar attached to the trailer yet, just grounded, so I may try hooking the connector up to the truck and probing around with a small jumper and see what I can scare up on there. It would be an interesting experiment and I may even get my 13 yr old's brain thinking in another direction than 4 wheelers and girls....
NAAAHHHHH, that'd never work! :confused2::confused2: